The Times has a special feature about On the Origin of Species, including annotations from various Scientists commenting on favorite passages. The first annotation is by the famous primatologist Frans de Waal, who comments one of the passages that struck my interest when I read the book earlier this year:

I should premise that I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny

Darwin, in the course of his opus, did not merely lay out the voluminous evidence for natural selection, he also frequently dropped hints of further research to come: for instance, the topic of cooperation. As de Waal notes, we're generally taught to think of Darwinianism as survival of the strongest individual competitors. Look at so-called "social Darwinism," and you'll see something that Darwin would likely have hated. Because he clearly believed that cooperation and education (an extension of "leaving progeny") were key components of the general competition between traits (evolution) in a population — that evolution is more than just my biceps are bigger than yours, my brain case has more volume than yours, etc.